Question

Rank the following atoms in the order of decreasing electron affinity. Li, Si, Mg, S

Answer 1

The energy released when a electron is added to a gaseous atom its ground state to form a gaseous negative ion is defined as the first electron affinity. The symbol is EA, and the unit is kJ/mol

electron affinity.

Li= 60 KJ /mol

Si=133.6 KJ /mol

Mg=-

S=200 KJ /mol

decreasing order of electron affinity

S>Si>Li>Mg

The first electron affinities of elements are negative in general except the group VIII and group IIA elements. Because the electron falls into the electrostatic field which is yielded by the atomic nucleus of a neutral atom, and lead to the potential reduction, the energy of this system is reduced. The second electron affinities of all elements are positive. This is because the negative ion is a negative electric field. And if now the other electrons enter the negative field, energy has to be applied to the system to overcome the repulsion that the negative electric field interacts with the electrons. According to the elements of one period, EA generally decreases from the left to the right across the periodic table. According to the elements of one main group, EA generally increases from the top to the bottom of the periodic table